1. Field of the Invention
This invention relates to a solid state laser, and more particularly to a solid state laser which utilizes a novel laser medium.
2. Description of the Prior Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a laser diode pumped solid state laser where a solid state laser medium doped with a rare earth element such as Nd is pumped with a semiconductor laser. As the solid state laser medium, a crystal of YVO.sub.4, YAG(Y.sub.3 Al.sub.5 O.sub.12) or the like doped with Nd has been widely used. In some of the laser diode pumped solid state lasers of this type, a bulk single crystal of nonlinear optical material which converts the wavelength of the solid state laser oscillation beam is provided in the resonator and the solid state laser oscillation beam is converted into second harmonic wave or the like in order to obtain a laser beam of a shorter wavelength. See U.S. Pat. No. 5,124,999, "Laser Research" vol. 18, No. 8, (1990), pp. 94 to 99 and the like.
For example, in "Laser Research" vol. 18, No. 8, (1990), page 646, there is reported an experiment where a 12.8 mW second harmonic output of substantially single longitudinal mode was obtained for a 760 mW semiconductor laser input power by converting a beam oscillated by Nd:YVO.sub.4 (Nd ion concentration of 1.1 at %) into second harmonics with a KTP crystal disposed in the resonator. In another report in the abstracts for "Laser/Atomic Oscillator and Ultimate Light Quantum Engineering" Symposium (promoted by Applied Physical Society and Quantum Electronics Society) E-2, (1990), p. 56, there is reported an experiment where a 9.1 mW second harmonic output of single longitudinal mode was obtained for a 740 mW semiconductor laser input power by use of Nd:YVO.sub.4 (Nd ion concentration of 2.02 at %) and a 7.0 mm thick KTP crystal.
The conventional solid state lasers are disadvantageous in that the Nd ion concentration can be increased only about 2 at % at most and accordingly absorption of the pumping laser power is small. When absorption of the pumping laser power is small, a multimode oscillation occurs due to a space hole burning effect, whereby output of the solid state laser becomes unstable.
Further in the conventional solid state lasers, since the absorption spectrum width for the pumping laser is narrow, there has been a problem that the output of the laser is apt to fluctuate with fluctuation of the wavelength of the pumping source such as a laser diode.